Refactor: Remove KeyedAccounts (2)

[Lichtso]

Problem

Continuation of #22226 as there are still many use sites of KeyedAccount to be replaced.

Summary of Changes

Removes use sites of KeyedAccount in tests of:

• invoke_context.rs
• message_processor.rs
• bank.rs
• BPF serialization

Also, adds InstructionContext::get_signers()
and improves error messages when modifying BorrowedAccounts.

Fixes #

[codecov]

Codecov Report

Merging #22274 (c6b145a) into master (1460f00) will increase coverage by 0.0%.
The diff coverage is 93.2%.

@@           Coverage Diff           @@
##           master   #22274   +/-   ##
=======================================
  Coverage    81.0%    81.1%           
=======================================
  Files         523      523           
  Lines      146793   146781   -12     
=======================================
+ Hits       119044   119090   +46     
+ Misses      27749    27691   -58